#include <iostream>
using namespace std;

// Definition for singly-linked list.
struct ListNode {
    int val;
    ListNode *next;
    ListNode() : val(0), next(nullptr) {}
    ListNode(int x) : val(x), next(nullptr) {}
    ListNode(int x, ListNode *next) : val(x), next(next) {}
};

ListNode* reverseList(ListNode* head) {
    if (head == nullptr || head->next == nullptr) {
        return head;
    }

    ListNode* p = reverseList(head->next);
    head->next->next = head;
    head->next = nullptr;
    return p;
}
